The main issue they used to have with MCreator is that you'd get a lot of low effort, repeated mods. "Ruby armor pack" "emerald tools" etc. MCreator is fine as long as you put thought and effort into the mod you're building through it. If it's something that is a little more basic than what they'd like, perhaps share it to the forums rather than a submission, or ask a site moderator for their opinion first before posting as a submission. From experience; it never hurts to ask them :)